Fix panic on send to closed channel during snapshot shutdown

[Ady0333]

Type of change

• Bug fix

Description

This PR fixes a shutdown race in the ledger snapshot manager that could cause a peer panic.

Snapshot generation runs asynchronously in background goroutines. If the peer shuts down while a snapshot is still in progress, snapshotMgr.shutdown() could close internal channels before those goroutines finish, leading to a send on closed channel panic.

The fix coordinates shutdown with in-flight snapshot goroutines so shutdown is safe even when snapshot generation is ongoing.

Additional details

The snapshot manager now signals shutdown to background goroutines and waits for them to exit before closing channels.

Tested locally with go test ./core/ledger/kvledger/....
Also ran go test ./core/ledger/kvledger -run Snapshot -v

Release Note

Fixes a shutdown race in the ledger snapshot manager that could cause a peer
panic when snapshots are generated during shutdown.

[Ady0333]

Hi @corwin @denyeart @yacovm ,
This PR fixes the shutdown panic and snapshot tests pass locally.
Please let me know if anything else is needed. Thanks!